Electric power-steering apparatus

ABSTRACT

An electric power steering apparatus including a rack shaft ( 21 ) formed with a first rack ( 27 ) meshing with a first pinion ( 25 ) connected to a steering shaft ( 3 ) and with a second rack ( 43 ) meshing with a second pinion ( 41 ) driven by a power assist mechanism, wherein the power assist mechanism includes an electric motor ( 31 ) and power transmission means for transmitting a driving force of said electric motor to the second pinion ( 41 ), and the electric motor is disposed substantially in parallel with the rack shaft ( 21 ).

TECHNICAL FIELD

The present invention relates generally to a rack assist type electricpower steering apparatus of a dual pinion type, and more particularly toa technology contrived to downsize the apparatus in a compactconfiguration.

BACKGROUND ARTS

Over the recent years, there has been progressed development of anelectric power steering apparatus (which will hereinafter be referred toas an electric power steering system abbreviated to EPS) using anelectric motor as a power source in a steering system for an automobile.The EPS has no direct driving loss (which is an engine driving lossrelated to a hydraulic pump) of an engine because of using an on-vehiclebattery as a power source of an electric motor, and can restrain atraveling fuel consumption down to a low level because of the electricmotor being driven only when performing steering assist.

A steering gear for a passenger car mainly adopts a rack-and-pinion typeat the present time because of its being high of rigidity and light ofweight, etc. Then, the EPS for the rack-and-pinion type steering gearadopts, in addition to a column assist type for driving a steeringshaft, a pinion assist type for driving a pinion meshing with a rackshaft, a rack assist type for driving the rack shaft itself, and so on.The pinion assist type EPS has a construction, wherein the electricmotor is normally mounted inside or outside a steering gear boxinstalled at a lower side end of the steering shaft, and drives thepinion in accordance with steering torque.

The conventional pinion assist type EPS has, however, such problems thatthe steering gear box unit increases in its size due to the installationof the electric motor, and it is difficult to mount the EPS and lay outthe engine, auxiliary machinery, etc. within the engine room, and soforth. Such being the case, there is proposed a dual pinion type EPS inwhich a pinion driven by the power assist mechanism is providedseparately from the pinion connected to the steering shaft, and thepower assist mechanism (i.e., the electric motor) can be therebydisposed in a desired position on the rack shaft (refer to, e.g.,Japanese Utility Model Publication No. 02-021346 (p. 2, FIGS. 1 and 2)).

In the dual pinion type EPS disclosed in Japanese Utility ModelPublication No. 02-021346 (p. 2, FIGS. 1 and 2), the electric motor isdisposed substantially coaxially with the pinion, and its axis has acomparatively large angle to the rack shaft. Hence, it follows that aspace in an upper portion of the power assist mechanism is occupied bythe protruded electric motor, and there still exist the problems that itis difficult to mount the EPS and lay out the engine, the auxiliarymachinery, etc. in the engine room, and so forth. Moreover, in the caseof taking the construction as in Japanese Utility Model Publication No.02-021346 (p. 2, FIGS. 1 and 2), a worm speed reduction mechanism havinga large speed reduction ratio can not be adopted, and there arises suchproblems that it is difficult to select the electric motor, the numberof constructive members rises, and so on.

DISCLOSURE OF THE INVENTION

It is an object of the present invention, which was devised under suchcircumstances, to provide a dual pinion type electric power steeringapparatus contrived to decrease its size in a compact configuration inorder to improve mountability and a degree of freedom of layout ofperipheral devices.

To accomplish the above object, according to the present invention, anelectric power steering apparatus includes a rack shaft formed with afirst rack meshing with a first pinion connected to a steering shaft andwith a second rack meshing with a second pinion driven by a power assistmechanism, wherein the power assist mechanism comprises an electricmotor and power transmission means for transmitting a driving force ofthe electric motor to the second pinion, and the electric motor isdisposed substantially in parallel with the rack shaft.

According to the present invention, the electric motor does not protrudefrom the rack housing that houses the rack shaft. Hence, themountability of the electric power steering apparatus on a vehicle isimproved, and simultaneously layout of an engine, auxiliary machinery,etc. is facilitated.

Further, in the electric power steering apparatus according to thepresent invention, it is preferable that the power transmission means isa worm speed reduction mechanism. In this case, a large speed reductionratio is easily obtained, and selection of the electric motor isfacilitated. In addition, the number of constructive members becomessmaller than in the case of using other speed reduction mechanisms.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing a portion, on a car-room side, of asteering apparatus according to the present invention;

FIG. 2 is a rear view of a dual pinion type EPS according to a firstembodiment of the present invention;

FIG. 3 is an enlarged view of a portion A in FIG. 2; and

FIG. 4 is a sectional view taken along the line B-B in FIG. 3.

EMBODIMENT OF THE INVENTION

One embodiment of the present invention will hereinafter be describedwith reference to the drawings.

FIG. 1 is a perspective view showing a portion, on a vehicle compartmentside, of an electric power steering apparatus according to theembodiment. A member designated by a numeral 1 in FIG. 1 represents asteering column which rotatably supports an upper steering shaft 3. Asteering wheel 5 is attached to an upper end of the upper steering shaft3, while a lower steering shaft 9 is connected a lower end of the shaft3 through a universal joint 7.

A steering gear mechanism constructed of a rack-and-pinion mechanism, apower assist mechanism, etc. is further connected to a lower end of thelower steering shaft 9. In FIG. 1, the numeral 13 designates a columncover for covering the steering column 1, and the numeral 15 denotes tierods connected to right and left side ends of the steering gear 11.

FIG. 2 is a rear view of the steering gear 11 according to the presentembodiment. In FIG. 2, a rack-and-pinion mechanism 22 and a power assistmechanism 23 are housed in a steering gear case (rack-and-pinionhousing) 21. The rack-and-pinion mechanism 22 is constructed of a firstpinion 25 connected to the lower steering shaft 9 and a rack shaft 29formed, on a right side in FIG. 2, with a first rack 27 meshing with thefirst pinion 25. The rack shaft 29 is so provided as to be movablebilaterally in FIG. 2 within the rack-and-pinion housing 21.

The power assist mechanism 23 is, as shown in FIG. 3 (an enlarged viewof a portion A in FIG. 2) and FIG. 4 (a sectional view taken along theline B-B in FIG. 3), constructed of an electric motor 31 that isdrive-controlled by a unillustrated control device, a worm speedreduction mechanism including a worm 35 connected to a shaft 33 of theelectric motor 31 and a worm wheel 37, and a second pinion 41 fixed tothe worm wheel 37. The second pinion 41 meshes with a second rack 43formed on a rack shaft 29.

The electric motor 31 is integrally fitted to the rack-and-pinionhousing 21 in parallel with the rack shaft 29. To be specific, theelectric motor 31 is disposed so that a rotary output shaft 33 of themotor 31 is parallel with the rack shaft 29, and the worm 35 connectedto the rotary output shaft 33 is also so rotatably supported in thehousing 21 via bearings 34 a, 34 b as to be parallel with the rack shaft29. A shaft member 37 b of the worm wheel 37 meshing with and thusdriven by the worm 35 extends at a right angle to a direction in whichthe rack shaft 29 extends. A second pinion 31 is fixed to the shaftmember 37 b and thus meshes with the second rack 43 of the rack shaft29.

In this embodiment, the second pinion 41 is disposed substantiallyperpendicularly to the rack shaft 29, and the worm wheel 37 fixed to thesecond pinion 41 rotates within the horizontal plane. Further, the worm35 meshing with the worm wheel 37 is disposed substantially in parallelwith the steering gear case 21 (i.e., the rack shaft 29 supportedtherein), and the electric motor 31 including the shaft 33 to which theworm 35 is fixed is also disposed substantially in parallel with therack shaft 29.

The operation in the present embodiment will hereinafter be described.

When a driver rotates the steering wheel 5, a rotational force thereofis transmitted to the steering gear mechanism 11 via the upper steeringshaft 3 and the lower steering shaft 9. The steering gear mechanism 11has the built-in rack-and-pinion mechanism constructed of the firstpinion 25 for converting a rotational input into a rectilinear motionand of the first rack 27, and hence the rack shaft 29 moves in any oneof the right and left directions. Then, a steering angle of travelingwheels changes through the right-and-left tie rods 15, whereby steeringis conducted.

Simultaneously, within the power assist mechanism 23, the electric motor31 rotates with predetermined rotational torque in any one of forwardand reversed directions on the basis of an output of an unillustratedsteering torque sensor. Thereupon, a rotational force thereof istransmitted to the rack shaft 29 through the worm speed reductionmechanism including the worm 35 and the worm wheel 37, the second pinion41 and the second rack 43 meshing with the pinion 41, and the rack shaft29 is energized in any one of the right and left direction in FIG. 2,thereby actualizing the steering assist.

On the other hand, the steering gear mechanism 11 according to thepresent embodiment takes such a configuration that the electric motor 31is disposed substantially in parallel with the rack shaft 29. With thisconfiguration, an amount of upward protrusion of the electric motor 31from the steering gear case 21 is by far smaller than that in JapaneseUtility Model Application No. 02-021346, etc., whereby mountability on avehicle and degree of freedom of layout of peripheral devices arelargely improved. Moreover, the electric motor 31 is connected via theworm speed reduction mechanism to the second pinion 41, and it istherefore possible to obtain a large speed reduction ratio with acomparatively simple construction and to actualize a decrease in thenumber of constructive members.

The discussion on the specific embodiment comes to an end so far,however, the mode of the present invention is not limited to theembodiment given above. For example, the electric motor and the secondpinion are connected via the worm speed reduction mechanism in theembodiment discussed above, and may also be connected via paired screwgears, etc. Further, the electric motor is disposed at the obliquelyupper portion of the rack shaft in the embodiment discussed above andmay also be disposed, e.g., just laterally of the rack shaft. Forothers, the whole configuration of the dual pinion system EPS, etc., isnot limited to the exemplification in the embodiment discussed above andcan be properly changed by a request, etc. in terms of the design orspecifications within the range that does not deviate the gist of thepresent invention.

As discussed above, according to the electric power steering apparatusof the present invention, the electric motor does not largely protrudefrom the rack housing that houses the rack shaft. Hence, themountability of the electric power steering apparatus on the vehicle isimproved, and simultaneously the layout of the engine and auxiliarymachinery, etc is facilitated.

1. In an electric power steering apparatus including a rack shaft formedwith a first rack meshing with a first pinion connected to a steeringshaft and with a second rack meshing with a second pinion driven by apower assist mechanism, an improvement characterized in that said powerassist mechanism comprises an electric motor and power transmissionmeans for transmitting a driving force of said electric motor to saidsecond pinion, and said electric motor is disposed substantially inparallel with said rack shaft.
 2. An electric power steering apparatusaccording to claim 1, wherein said power transmission means is a wormspeed reduction mechanism.